Gate valves are typically used when a straight-line flow of fluid and minimum flow restriction are required. They may also be used in christmas trees used for oil and gas extraction. Typically, the gate has body with a cavity and a flow passage extending through the body and intersecting the cavity to allow flow through the valve. When the valve is wide open, the gate is drawn into an end of the valve cavity away from the flow passage. The flow passage is typically the same size as the pipe in which the valve is installed.
A typical gate valve used in connection with oil and gas production has a flow passage that intersects a central cavity in the valve. Seat rings are placed in counterbores formed in the flow passage at the intersection of the flow passage with the cavity. An obstruction or gate is moved past the seats between open and closed positions to seal the cavity from the passage.
The seats generally have seals which seal the seat to the counterbore of the flow passage. These seals are typically elastomeric seals and when located on the downstream seat prevent the entry of fluid from the central cavity or chamber of the body to the downstream flow passage. Seals located on the upstream seat can act as a check valve to fluid flow. For gate valves designed with unidirectional sealing when the gate is closed, fluid will flow past the upstream seat into the chamber or cavity of the body. The fluid pressure in the chamber is sealed by the seal of the downstream seat formed between the gate and the seat. In addition, a sand screen may also be positioned in the seats to protect the valve from sand intrusion. For gate valves designed with bidirectional sealing when the gate is closed, fluid is maintained on one side of the gate and not allowed to flow into the chamber or cavity of the body.
Typically, there is a small amount of movement possible for the seat, resulting in axial movement of the seal as the valve opens and closes. This axial movement results in seal wear. When gate valves are subjected to high pressure environments, creep and yield can result in elastomeric seal wear. This results in a life-limited seal. The elastomeric seal also has temperature limits that prevent use at the highest temperatures seen in oil and gas fields. To counter this problem, a Teflon-type elastomeric seal has been used. The seal is pressure actuated to seal when in the downstream seal position and loaded from the direction of the valve cavity, as shown in FIG. 1. When in the upstream seat position, the pressure load from the pipe direction tends to collapse the seal at a relatively low pressure once the seal is overcome. A T-shaped insert prevents that collapse from resulting in damage to the spring. However, this configuration does not address the wear associated with elastomeric seals.
A need exists for a technique to increase life of seals in gate valves by reducing wear in the seat seal.